1. Field of the Invention
The present invention relates to an image input apparatus for obtaining an object image of a fingerprint, rubber stamp, or the like having irregularities (pits and projections).
2. Related Background Art
Known as this kind of image input apparatus, for example, are apparatus (first conventional example) disclosed in U.S. Pat. No. 4,932,776 and Japanese Patent Application Laid-Open No. 6-300930. As shown in FIG. 1, such an image input apparatus comprises a fiber optic plate (FOP) B in which a number of optical fibers A are bundled together and a light source (illuminating means) D for irradiating an entrance surface C of the fiber optic plate B, thereby outputting an image of a fingerprint or the like having irregularities. In this image input apparatus, when a finger C1 or the like comes into contact with the entrance surface C of the fiber optic plate B, light from the light source D is made incident on the fiber optic plate B only from protruded portions of the finger C1 which are directly in contact with the entrance surface C. Then, the incident light propagates through the fiber optic plate B so as to be outputted from an exit surface E, thereby yielding an image El (fingerprint image or the like) corresponding to the irregularity pattern of the fingerprint or the like.
In this image input apparatus, however, if light is incident from a part where the finger C1 or the like is not in contact with the entrance surface C, the image E1 to be outputted will have a lower contrast and become unclear. Accordingly, in general, the entrance surface C of the fiber optic plate B is inclined with respect to the optical axis (coinciding with the optical axis direction of each optical fiber therein) by a predetermined angle. In particular, such a fiber optic plate (first conventional example) having the entrance surface C inclined with respect to the optical axis has been known as slant FOP, and its angle of inclination .theta..sub.0 has been referred to as slant angle. In the fiber optic plate B of the first conventional example, the slant angle .theta..sub.0 and the numerical aperture NA of the optical fibers A are matched with each other in order to prevent the unnecessary light (stray light) incident thereon from the part not in contact with the entrance surface C (depressed portion of the finger or the like not in contact with the entrance surface C) from propagating through the optical fibers A (from satisfying a total reflection condition). As a result, the above-mentioned unnecessary light is not theoretically outputted from the exit surface E.
Also, for example, Japanese Patent Application Laid-Open No. 7-174947 discloses, as a fiber optic plate of a second conventional example, a structure in which an optical absorber is disposed between optical fibers constituting the fiber optic plate. As this optical absorber is disposed, light is prevented from traveling across the neighboring optical fibers, thereby the unnecessary light incident on the fiber optic plate from the part where the object C1 and the entrance surface C are not in contact with each other is efficiently attenuated in the fiber optic plate. Accordingly, in the fiber optic plate having such a structure, the contrast of the image outputted therefrom can be improved.